1. Field of the Invention
The present invention relates to ignition controllers for gas-fired forced combustion air heating appliances.
2. Description of the Prior Art
To obtain higher efficiency, many gas-fired furnaces use induced draft combustion air blowers and electronic fuel ignition. An important consideration for any ignition controller for a gas-fired heating appliance is safety in the case of a failure of one or more of the components of the appliance and its control system. For example, Underwriters Laboratory requirements for failure modes and effects analysis (FMEA) require that all single component failures either cause a safe shutdown of the appliance or, if undectected, the ignition control must continue to operate safely. If the ignition control continues to operate after a first order component failure, then for all combinations of second order failures, it must either continue to operate safely, or shutdown safely. This is a significant constraint when inputting a safety related variable control value such as an ignition time duration into an electronic ignition control.
Several potential problems exist with standard control systems for induced draft forced combustion air furnaces when considering the duration of an ignition light timing with an electronic ignition control. First, with analog controls, different resistor and capacitor values must be selected during the manufacturing of the control so that the final device operates with only a single fixed timing.
Second, with digital controls using microprocessor (MPU) based devices, the type of control manufacturing process used for analog controls is not normally used because it requires a different logic design for each timing desired, which is costly and takes away control flexibility. The normal method when using a digital control is to input the variable ignition timing value digitally through the digital control's input pins. However, this approach uses a large number of MPU input pins if the input variable value has a significant number of options. For example, if the ignition timing variable value has five options, then four input pins are required to input a fail safe code so that no single component goes undetected.
Third, the use of a digital control having an analog-to-digital convertor is a method for inputting a variable with a large number of options, without using a large number of device input pins, at low cost, and without restricting flexibility. One way to satisfy the safe failure modes requirement is to input the same ignition timing information through a second analog-to-digital input. However, this method detects only external control component failures, and does not address any internal failures in the controller logic.